The present invention relates to a control system for continuously controlling a driving force distribution between front and rear wheels of a four wheel drive vehicle, and more specifically to a driving force distribution control system which can provides a superior control performance during starting of the vehicle.
Japanese Patent Provisional (unexamined, KOKAI) Publication No. 63-203421 shows a conventional example. A driving force distribution control system of this conventional example is designed to increase an engagement force of a transfer clutch to increase a driving force transmitted to the front wheels in accordance with a front and rear wheel speed difference when a drive wheel slip is produced during acceleration or driving on a slippery road.
However, this conventional system has only one control mode in which the controlled variable is a function of the front and rear wheel speed difference, and always relies on this control mode irrespective of vehicle operating conditions. When the accelerator pedal is depressed quickly to start the vehicle speedily, the sudden depression of the accelerator pedal increases the driving force transmitted to the rear drive wheels, and causes wheel spin of the rear wheels. In such a case, the conventional system cannot increase the clutch engagement force of the transfer clutch until the detected front and rear wheel speed difference is actually increased. In rapid starting, this conventional system cannot prevent drive wheel spin sufficiently and cannot provide satisfactory driving performance. The delay of the control action is further increased by delay in detection of the front and rear wheel speed difference, and delay in the response of a hydraulic unit.
Furthermore, the conventional system forces the transfer clutch to undergo continuously an excessively increased driving force in a slippingly engaged state, so that the transfer clutch is subject to severe wear, and seizure of the clutch plates.